(1) Field of the Invention
The present invention relates to time modulated fluidically actuated control mechanisms. More specifically, this invention is directed to a technique which permits the use of such time modulated control mechanisms in environments where the natural frequency of the control mechanism and its load is comparatively high. Accordingly, the general objects of the present invention are to provide novel and improved methods of such character.
(2) Description of the Prior Art
Time modulated control mechanisms are well known in the art. An example of an open loop control mechanism of the time modulated type may be found in U.S. Pat. No. 3,430,536. A similar control mechanism, which is of the closed loop type, is shown and described in U.S. Pat. No. 3,521,535. The disclosures of these two patents are incorporated herein by reference and a full discussion of time modulation as it applies to fluidically actuated control mechanisms will be found in U.S. Pat. No. 3,430,536.
As pointed out in U.S. Pat. No. 3,430,536, and particularly with reference to the description of FIG. 7 of the patent, it has previously been considered necessary to operate time modulated pneumatically actuated control mechanisms with a carrier frequency f.sub.c above the critical range of system natural frequencies; the "dither" amplitude of the control being acceptably small above system resonance due to attenuation of the pressure variations resulting from cycling of the control valve or valves at the carrier frequency. Operation with such a "super-critical" carrier frequency, typically in the range of 80 Hz to 175 Hz, covered most previous control applications. It has been widely believed in the art that operation with a carrier frequency below the range of resonant frequencies of the control system; i.e., operation with a "sub-critical" carrier; would result in a prohibitively large dither amplitude. As used herein, the term "dither" refers to load motion at the carrier, rather than the command, frequency.
To summarize the above, referring again to FIG. 7 of U.S. Pat. No. 3,430,536, a principal objective in the design of time modulated pneumatically actuated control mechanisms is to permit changes in load with a minimum effect on the dither amplitude. The graphical showing of FIG. 7 of U.S. Pat. No. 3,430,536 would indicate to those skilled in the art that this objective could be achieved only by operating with a "super-critical" carrier frequency above the critical range of system natural or resonant frequencies.
The above principals will apply to and do not impede most control applications. There are, however, certain control applications where the load has small inertia and the system natural frequency range is thus considerably higher than previously experienced. In such applications, for example where the system resonant frequency range exceeds 175 Hz, there are substantial impediments to operation with a super-critical carrier frequency. Thus, for example, above approximately 175 Hz the electrical power required for the necessary fast response of an economical solenoid becomes prohibitively large. In fact, for most control applications wherein the system natural frequency range extends above 175 Hz, solenoids which meet system requirements for volumetric efficiency and weight are unavailable. Additionally, as may be seen by reference to FIG. 7 of U.S. Pat. No. 3,430,536, as carrier frequency is increased the dither amplitude ratio decreases to a very small value. While a large dither is obviously unacceptable, experience has shown that small amplitude load motion at a carrier frequency may improve overall control system efficiency by reducing the frictional forces which have to be overcome to reposition the control mechanism in response to varying input command signals.